Method of treating electrodes for use in electroplating baths



R. D. BED! Nov. 11; 1969 METHOD OF TR'EATINGIELECTRODES FOR USE IN. ELEGTROPLATINGBATHS Filed Dec. 23, 1968 u u 5 O u w o o AU I FIG. I

FIG. 2

United States Patent 3,477,920 METHOD OF TREATING ELECTRODES FOR USE IN ELECTROPLATING BATHS Ram Dev Bedi, Southfield, Mich., assignor to M&T Chemicals Inc., New York, N.Y., a corporation of Delaware Continuation-impart of application Ser. No. 291,917, July 1, 1963. This application Dec. 23, 1968, Ser. No. 786,040

Int. Cl. C23b 5/48 US. Cl. 204--15 15 Claims ABSTRACT OF THE DISCLOSURE In accordance with certain of its aspects, this invention relates to the process for electroplating a plate from a bath onto a cathode having predetermined areas to be plated and selected areas to be stopped-off which comprises placing said selected areas in intimate electrical contact with a metal in low hydrogen overvoltage state having a hydrogen overvoltage lower than that of the cathode of from 0.015 to 0.25 volt selected from the group consisting of platinum, palladium, rhodium, iridium, and nickel, and subjecting said cathode including said selected areas to plating current density whereby said plate is deposited onto said predetermined areas and said selected areas remain stopped-01f.

This application is a continuation-in-part of US. patent application Ser. No. 291,917, filed July 1, 1963 and now abandoned.

This invention relates to a method of treating electrodes. More particularly, this invention relates to a method of treating selected areas of electrodes to effect stoppingotf, i.e. to permit control of selected areas of an electrode so that they may be free from plate which is to be deposited on other areas of the electrode.

As is well known to those skilled in the art, metals may be immersed in various baths in the course of different treating processes wherein desired portions of the metal may, for example, be subjected to electroplating. As is well known to those skilled in the art, electroplating of various metals, typically nickel, copper, tin, zinc, or chromium may be efiected in baths of varying acidity and composition; many ofthese baths may contain chloride or fluoride which may be an integral part of the bath or present as an additive. Because of differences in areas or geometry or accessibility, there may be predetermined portions of the cathode which are to be plated and selected areas on which a plate is not desired. Specifically, the surface of these selected areas, if not stopped-elf, i.e. if not properly treated to prevent plating thereon, may undesirably be plated sufficiently to alter the dimensions substantially and/or to spoil the appearance of the finished article. The problem of stop-off may be present in various baths, typified by chromium plating baths, nickel plating baths, acid copper baths, acid tin baths,, zinc baths containing halides or halide-complexes, etc.; for purpose of convenience, reference will hereinafter be made to chromimum plating baths.

Commonly the areas of the cathode on which plate is not desired may be covered or painted with a masking or stop-off material which prevents deposition and buildup of the plate on these high current density areas which would normally be plated. The masking or stop-oil coatings are commonly subjected to severe corrosive conditions for an extended period of time. For example, in chromium plating these materials may remain in position for extended periods of time, often for days, under the "ice most severe oxidizing conditions in contact with baths which may contain sulfuric acid and chromic acid together with numerous other chemicals. Furthermore, the stop-otf material must be resistant to the highly erosive effect of the hydrogen which is liberated.

As is well known to those skilled in the art, chromium plating for example may be effected by use of a bath containing chromic acid and sulfate together with other compounds which may be employed to efiect various desirable results. Typical of these additive compounds may be fluorides or fluoride complexes. Other illustrative chromium plating systems may include soluble catalyst systems containing e.g. chromic acid, and sulfate such as sulfuric acid in amount sufficient to give the desired concentration of sulfate ion, or self-regulating baths, typically those containing silicofluorides together with sulfate.

During chromium plating from baths typified by the foregoing, it is common to operate at a temperature which may vary depending upon the type and purpose of the plating operation. Commonly, however, the temperature of operation may be 34 C.-72. C. and typically 48 C.- 63 C. The cathodic current density may preferably be controlled to fall in the range of 8-90, and typically 12-45 amperes per square decimeter on the selected areas of high current density whereon plating may occur. However, because there may be certain selected high current density areas which are not to be chromium plated, it is desired to stop-ofi these areas.

In chromium plating systems, it has heretofore been common to coat with tapes or waxes those selected portions of the cathode where no plate is desired. The use of such techniques is time consuming and expensive. Tapes or waxes must be carefully placed and removed, since any holes or spaces (including edg-e areas which may be undermined) in the protective covering will allow severe etching of the so-exposed metal to take place. Such tapes or waxes may be expensive to purchase and apply. Various other attempts to stop-off selected areas of cathodes in the noted plating baths have not been uniformly successful, and there is today no economical stop-01f system which is completely satisfactory.

Stop-off materials to effect these results have heretofore been either paints or tapes. Typical paints may include plastic materials such as polymers of vinyl chloride or vinyl acetate which may contain plasticizers, pigments, solvents, etc. These may be applied by painting onto the desired portions of the workpiece which are not to be subjected to the action of the solution. Masking tapes which may be employed for the same purpose may be wrapped around the piece and cover the areas which are to be protected or stopped-oil.

As is well known to those skilled in the art, application of e.g. masking tapes and paints is a time-consuming and expensive job. Furthermore, many of the stop-off paints or tapes are unsatisfactory in that, during plating, they may become pervious to the solution or not adherent to the workpiece. Perviousness permits growth of dendritic trees at or adjacent to the tape or paint: such growths must be removed by grinding or machining to make the part satisfactory with respect to appearance and dimensionality. It is a particular objection that tapes and paints permit build-up of the plate at the point immediately adjacent thereto and thus may require .a grinding operation, after plating is complete, to remove the ridges which may be present. It is a further objection that paint or masking tape must be removed from the workpiece at the completion of the plating operation. This may typically be done by cutting, peeling. scraping, by action of solvents, etc.

It is an object of this invention to provide a novel technique for masking or stopping-off areas to be maintained free of a deposited plate during plating. Other objects will be apparent to those skilled in the art on inspection of the following detailed description.

In accordance with certain of its aspects, the process of this invention for electroplating a plate metal from a bath onto a cathode having predetermined areas to be plated and selected areas to be stopped-off comprises placing said selected areas in intimate electrical contact with a metal in low hydrogen overvoltage state having a hydrogen overvoltage lower than that of the cathode of from 0.015 to 0.25 volt selected from the group consisting of platinum, palladium, rhodium, iridium, and nickel, and subjecting said cathode including said selected areas to plating current density, whereby said plate metal is deposited onto said predetermined areas and said selected areas remain stopped-off.

Typical of the plate metal with which the process of this invention may find use may be the aforementioned chromium plate including hard chromium plate. This invention will be especially suitable for use with the noted chromium plating baths, including the soluble or sparingly soluble catalyst-containing chromium plating baths.

The cathodes which may be used in the practice of this invention may be those basis metal articles upon which a plate, typically a chromium plate, is desired. Typically these basis metals may be characterized by their high hydrogen overvoltage, typically 0.3 volt to 1.2 volts and preferably 0.7 volt to 1.00 volt. Hydrogen overvoltage is a measurement of the amount of work required to liberate hydrogen at the cathode. Thus, it is conventionally expressed as the difference between the potential of the electrode when hydrogen is liberated during electrolysis and the potential of the reversible hydrogen electrode, both potentials being referred to the same electrolyte. A cathode having a lower hydrogen overvoltage will, therefore, generate hydrogen at a lower voltage than a cathode having a high hydrogen overvoltage. High hydrogen overvoltage basis metals may include ferrous metals such as iron or steel, including stainless steels, low carbon steels, nickel steels, chromium steels, chromium-nickel steels, etc., particularly when these metals are in bright, solid highly polished conditions. The basis metal may also be a non-ferrous metal including nickel, copper, brass, zinc, aluminum, etc.

In the practice of this invention, the selected areas of the cathode on which it is not desired that plating occur may be placed in intimate electrical contact e.g. coated, with a low hydrogen overvoltage metal selected from the group consisting of platinum, palladium, rhodium, iridium, and nickel. Such metals in their low hydrogen overvoltage form may be commonly characterized by finely divided surface condition. Low hydrogen overvoltage materials are described on page 116 of Reference Electrodes by Ives and Janz (1961) Academic Press, New York and values of hydrogen overvoltage by different methods are given on pages 1144-1145 of the Corrosion Handbook by H. Uhlig (1948) John Wiley & Sons. Typically, such low hydrogen overvoltage materials may have values of 0.015 volt to 0.25 volt and preferably 0.015 volt to 0.10 volt.

These metals may be placed in intimate electrical contact with those selected areas of the surface to be stoppedoff. They may be employed in connection with the cleaned or prepared surface of the basis metal in a number of ways. One highly convenient method for applying the metal to the selected areas is by deposition, electrolytically, chemically, or by immersion, from a solution containing an ion of the metal. This coating or deposition may be accomplished e.g. by spraying, contacting, brushing, dipping, electroplating, immersion plating by the well-known Electroless nickel technique, etc. When the ions of the metal come into intimate contact with the basis metal, the former may be reduced to chemically form a deposit of the metal on the surface of the basis metal: and such deposits may typically be in the form of finely divided metal, e.g. black metal such as black platinum, palladium, or nickel.

Compounds of the metal including salts, acids, etc. may also be employed in the process of this invention. For example, chloroplatinic acid solutions may be employed as a source of platinum ions. Similar equivalent metal compounds may be employed. When the metal may exist in more than one oxidation or valence state, any of these may generally be employed. Typical ionic metal compounds which may be used to obtain metal deposits which may preferably find use in the practice of this invention may include palladium dichloride, chloroplatingic acid, platinum chloride, platinum diamine dinitrite, potassium chloroplatinate, potassium chloroplatinite, tetrammine platinous chloride, tetrammine platinous fluoride, palladium nitrate, rhodium chloride, iridium tetrachloride, chloroiridic acid, etc. Other deposits may be employed. The preferred compounds may include palladium chloride, rhodium chloride, and chloroplatinic acid in aqueous solutions. Solutions of these in organic solvents such as ethanol, propanol, acetone, benzaldehyde, ether, etc. may be employed.

In a preferred embodiment, when it is desired to chromium plate a steel piece without plating selected areas, these latter areas may be coated with e.g. platinum by brushing or by treating the surface in question as by dipping the piece into an aqueous (or a non-aqueous) solution of a platinum salt complex e.g. the chloride, PtCl which may be present as an alkali metal salt thereof e.g. Na PtC1 Typically an aqueous solution containing at least about 0.01 g./l. up to the limit of solubility, and typically 3 g./l. of complexed platinum metal added as chloroplatinic acid, and 4-350 g./l., preferably 20-80 g./l. of sodium chloride may be used. It is a particular feature of this invention that the presence of sodium chloride in the noted platinum solution permits a high degree of control of rate of deposition of platinum; it also imparts to the deposit a characteristic dark color which facilitates application; and it imparts a greater degree of adhesion of the platinum. The basis metal may be painted with this solution which may be allowed to stay thereon for typically 15-30 seconds. During this period, finely divided platinum metal may coat the basis metal.

When the selected areas of the basis metal have acquired a stop-off coat, the solution of the latter may be removed as by washing or rinsing with water, for example: the so-treated cathode is then ready for further processing.

In another embodiment, those portions of a steel cathode which may be selected areas on which it is not desired to have a plate may be dipped into a solution of palladium ions e.g. palladium chloride (having a concentration of at least about 0.01 g./l. up to 25 g./l., and preferably in amount of 4 g./l.) and immersed for at least about 1-2 seconds, after which time the cathode may be removed from the solution, rinsed, cleaned in the usual manner, and placed in the plating tank. When the desired thickness of plate metal, e.g. hard chromium plate, has been achieved, the cathode may be removed from the bath; the stopped-off selected areas which had been treated by the process of this invention will then be found to be free of plate.

In accordance with another embodiment of the invention, a cathode (typically of steel) may be treated by brushing or spraying thereon a solution of the metal salt, and allowing the solution to remain in contact with the surface of the cathode for a period of time sufliciently long (typically l-2 seconds) to insure adequate reaction.

In accordance with another embodiment of this invention, a novel stop-off composition for preventing plating on a high current density area of a high hydrogen overvoltage metal cathode may comprise a compound of a metal, which metal in finely divided form has a low hydrogen overvoltage, and a substantial excess of a pasteforming vehicle. Typical paste-forming vehicles may include pastes derived from water-soluble polymers (including their derivatives), such as starches, gums, carboxymethyl cellulose, pectin, alginates, polyvinyl alcohol, polyacrylamide, gelatin, clay (typically kaolin or bentonite), silica gel, caragheenates, etc. Other well-known equivalent paste-forming materials or thickening agents may be employed. The so-prepared paste may then be allowed to remain in contact with the cathode until reaction therewith is achieved, and the residual paste may then be removed by rinsing, rubbing, brushing, scrubbing, etc. When the paste method of treatment is employed, it will be particularly highly desirable to remove substantially all the residual material before plating to avoid poisoning of the low hydrogen overvoltage surface.

The immersion or contact time required to obtain adequate protection for the selected areas of the cathode will be largely dependent upon the concentration of the metal in the treating solution, or paste, the temperature at which treatment is carried out, the nature of the basis metal, etc. Typically the contact time may be of the order of at least 1-2 seconds. Shorter contact times may be employed, but the protection obtained thereby may not be as satisfactory. Longer contact times mayalso be used, although often no attendant advantages may be enjoyed by substantially longer periods.

The treating solutions or pastes which may be employed in the practice of this invention may be solutions or pastes containing a soluble ion of a metal selected from the group consisting of platinum, palladium, rhodium, iridium, and nickel. An concentration of the metal ion may be used from extremely low concentrations up to saturated solutions. However, if lower concentrations are employed, longer contact times and larger amounts of solution or paste may be required. If highly concentrated solutions or pastes are used, serious losses of the active ions present may occur because of drag-out. In order to produce a treating solution with the best balance of treating properties, it is preferable to employ the said metal in the equivalent metal concentration of at least about 0.1 g./l.

It is a feature of this invention that stop-off may be accomplished by placing the said low hydrogen overvoltage metal in intimate electrical contact in the solution with the selected areas of the cathode. This metal may be provided by means of a thin foraminous sheet, including mesh, expanded metal, perforated metal, etc. in intimate electrical contact with, and preferably positioned immediately adjacent to and electrically connected to, the cathode areas to be stopped-off. The foraminous sheet may be made of the metal or it may be metal (e.g. steel), coated with metals including platinum or palladium in the manner noted supra. The portion of the cathode to be stopped-off may preferably be overlaid with and contiguous to the thin foraminous sheet. Use of such a foraminous sheet immediately adjacent to and electrically connected to the cathode permits plating predetermined areas of the cathode while satisfactorily stopping-off selected areas.

The quantity of the stop-off metal which may be required to effectively stop-off the selected areas of the cathode may be very small. Larger amounts may be employed, but it may be found that such larger amounts, i.e. thicker layers, will not appreciably be better. Although the preferred amount of metal may be approximately the amount required to form a mono-molecular layer thereof on the area treated, it may be found that a continuous mono-molecular layer may not be formed. Instead, the metal may be present in the form of discrete islands and a considerable percentage of the treated area of the cathode may he apparently exposed to contact with the plating solution. It would be expected that such exposed areas would not be stopped-off and would be plated in the same manner as a totally untreated cathode. Surprisingly, it has been found that even these relatively non-uniform treatments are highly effective in stopping-off the selected areas of the cathode.

The stop-off film or deposit of metal may possess good adhesion to the cathode surface. When 'very thin films of metal are used, e.g. a sufficient amount to produce a mono-molecular layer or less, the treated surface may look substantially similar to an untreated surface of the same material. Sometimes the treated surface may appear somewhat darker and more lustrous than the untreated. If this be objectionable, then the stop-off metal may be removed by simple buffing after the plating operation.

It has been found to be particularly desirable to remove the solution or paste from the deposited film of metal, particularly when that film is of palladium or platinum. Preferably, this may be effected by a rinse with warm-tohot water.

A specific preferred solution which may be used in practice of this invention may contain 4 g. of palladium chloride, 20 g. of sodium chloride, hydrochloric acid sufficient to lower the pH to about 1.5, and water to make up one liter.

Another specific solution which may be used in practice of this invention may contain 2.5 g. chloroplatinic acid (H PtCl 6H O) and water to make up one liter.

Specific illustrative paste compositions suitable for use in practice of this invention may include the following:

Components: Parts by weight Silica gel Rhodium chloride 1.5

Water 1000 Silica gel 100 Palladium tetrachloride 3 Water 1000 Methocel brand of methyl cellulose 20 Platinum chloride 1 Water 1000 Agar 30 Chloroiridic acid 2.5

Water 1000 Polyvinyl alcohol (Swift Gelvatol brand) 100 Rhodium chloride 2 Water 1000 Gelatin (Keystone type C-L-X) 20 Chloroiridic acid 4 Water 1000 The following illustrative examples clearly point up the novel features of this invention.

EXAMPLE 1 To demonstrate the superior stop-off properties of the novel technique of this invention, a cylindrical piece of pipe of type 1015 steel, 19.2 cm. long and 4.9 cm. outside diameter, was degreased by treatment in a vapor-phase degreaser, and further cleaned by anodic cleaning in alkali, rinsing in water, dipping in 2% sulfuric acid, rinsing in water, and drying. An aqueous solution (at 25 C.) containing 4 g./l. of chloroplatinic acid, 40 g./l. of sodium chloride, and hydrochloric acid suflicient to lower the pH to 1.5 was carefully brushed over a selected area 5.04 cm. long at one end of the pipe. The solution was permitted to remain there for 5 seconds and was then rinsed off 7 with hot (70 C.) water. The treated area had a slight gray cast and was somewhat more lustrous than the untreated area. The other end of the pipe was stopped-off by using a standard plastic stop-off or masking tape-that sold as the 470 brand of stop-off tape manufactured by Minnesota Mining and Mfg. Co. This tape covered an area of the pipe 5.04 cm. in length.

The pipe was then made the cathode in a chromium plating bath containing 250 g./l. chromic acid, and 2.5 g./l. sulfate ion (as sulfuric acid). The anodes were standard insoluble lead alloy electrodes containing 4% tin. The outside surface area of the pipe was 294 square centimeters and the current density was 47.7 amperes per square decimeter, based on outside area. The pipe was plated for 5.25 hours while the bath was maintained at 57 C.58 C. At the end of this time, it was removed from the bath, rinsed and dried. The surface of the pipe was examined.

The attached FIG. 1 schematically shows the test piece 10 wherein the portion 11 is that portion which was treated, as noted supra, with the novel platinum composition of this invention and the portion 12 was masked off with the prior art masking tape. The portion 13 of the pipe was untreated except as hereinbefore noted. The vertical scale at the left indicated the thickness of chromium plate deposited on the test piece.

It will be observed that the thickness of plate deposited on the central portion B of section 13 is about 0.09 millimeter. At the end of section 13 immediately adjacent to end section 12 which had been treated by a prior art masking tape, it may be observed that the thickness of the plate undesirably increases in area C to a maximum of about three times the desired dimension. Not only is there such an increase, but there is a highly visible bead or ridge adjacent to the tape; and this bead or ridge of deposited metal must be removed by careful (and expensive) grinding. This bead may extend over an area reaching 23 centimeters or more away from the selected area which was covered by the tape. Although not plotted in FIG. 1, it was also observed that the chromium plate, in time, had deposited on the piece under the tape in the area to the right of area C. This undesired deposit must be removed as by electrochemical stripping in e.g. a pyrophosphate bath. This latter step may require a stoppingoff of the properly deposited chromium plate to prevent change of dimension of the plated portion.

It will also be observed that at the end of area A of section 13 adjacent to section 11 (which has been treated by the technique of this invention) there was no bead effect. The thickness of plate gradually dropped off. Furthermore, there was absolutely no plate on the selected area 11 which had been stopped-oflf.

It will be apparent to those skilled in the art that this novel technique insures a high degree of control over the plating operation and thereby minimizes or eliminates post treating operations including removal of masking or stopolf compositions and/or further treating of the piece to insure dimensional accuracy.

In other examples a similar piece may be treated with the compositions hereinbefore disclosed to effect similar results.

EXAMPLE 2 It is a feature of this invention that, if desired, it may be possible to obtain a plate which is substantially entirely of uniform thickness from one end of the predetermined area to the other. In this embodiment, the predetermined areas of the piece may be extended by an auxiliary predetermined piece (of dimensions similar to the predetermined area) placed immediately adjacent to the predetermined area of the piece, which in this embodiment may be the entire piece. There may be positioned next to the auxiliary predetermined area-piece a selected area-piece which has been treated according to the process of this invention. The auxiliary predetermined area-piece and the selected area-piece may if desired be difierent sections of a single piece of metal one part of which may have been treated in accordance with this invention While the other part may not have been so treated.

In FIG. 2 there is shown a typical embodiment of such a set-up which may make it possible to uniformly plate the entire predetermined area to a uniform plate thickness. In this embodiment, the test piece, e.g. a mandrel 20 having a predetermined area over its entire length 21 may be fitted with an auxiliary predetermined area-piece 22 which possesses substantially the same cross-section as piece 20. The so-fitted pieces 20 and 22 may then be further fitted with selected area piece 23 which has been treated i.e. stopped-off in accordance with this invention, as described in Example 1. For control, the other end of piece 20 may be fitted with blank pieces 24 and 25 which are identical respectively to pieces 22 and 23 except that piece 25 was not treated in accordance with this invention. These several pieces 22, 23, 24, and 25 may be held in place against piece 20 as by screws or by desired jigs or fixtures.

The so-assembled workpiece was then electroplated under conditions identical to those of Example 1 except that the bath employed contained 250 g./ 1. chromic aicd, 1.25 g./ 1. sulfate ion (as sulfuric acid), and 2.5 g./ 1. silicofluoride ion (as potassium silicofiuoride). The plated piece was examined at the conclusion of the experiment and the thickness determined as represented schematically in FIG. 2.

Itwill be observed that the thickness of the plate deposited on the central portion E of predetermined area 21, and the end thereof immediately adjacent to auxiliary predetermined area 22 was substantially uniform as was desired. Because of the presence of auxiliary selected area piece 23 and auxiliary predetermined area piece 22, there was no bead or ridge on the piece 20 in the noted areas. It will be noted that area D fell on the auxiliary predetermined area piece 22.

It was further observed that on the control end of piece 20 which bore pieces 24 and 25 (identical to pieces 22 and 23 except that piece 25 had not been treated in accordance with this invention) there was a bead or ridge over area F which was undesirable in that it would have to be removed from the finished piece as by grinding. Thus it will be observed that the novel technique of this invention permits attainment of a piece which has been plated uniformly over its entire length.

It is a further feature of this invention that it may be possible to obtain the noted advantages by treating conductive plating racks according to the novel technique of the invention. Such racks when treated may be used to plate pieces in manner to obtain a plate over the entire piece while simultaneously eliminating any deposit on the rack which might otherwise build up over a period of time, thick deposits which would interfere with control of the thickness of the deposit on the piece.

The invention is especially applicable to plating processes where the efficiency is substantially less than and where large volumes of hydrogen are evolved at the cathode simultaneously with the deposition of the plate metal. Such processes include nickel, copper, tin, and zinc plating, as well as chromium plating.

As many embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention includes all such modifications as come within the scope of the appended claims.

I claim:

-1. The process for electroplating a plate from a bath onto a cathode having predetermined areas to be plated and selected areas to be stopped-off which comprises placing said selected areas in intimate electrical contact with a metal in low hydrogen overvoltage state having a hydrogen overvoltage lower than that of the cathode of from 0.015 to 0.25 volt selected from the group consisting of platinum, palladium, rhodium, iridium, and nickel, and subjecting said cathode including said selected areas to plating current density whereby said plate is deposited onto said predetermined areas and said selected areas remain stopped-off.

2. The process for electroplating a plate as claimed in claim 1 wherein said metal is deposited onto said selected areas of said cathode by immersion plating.

3. The process for electroplating a plate as claimed in claim 1 wherein said metal is deposited onto said selected areas of said cathode by electrodeposition.

4. The process for electroplating a plate as claimed in claim 1 wherein said metal is deposited onto said selected areas of said cathode by chemical reduction.

5. The process for electroplating a plate as claimed in claim 1 wherein said plate is selected from the group consisting of chromium and nickel.

6. The process of electroplating chromium from a chromium plating bath onto a steel cathode having predetermined areas to be plated and selected areas to be stopped-off which comprises placing said selected areas in intimate electrical contact with a metal in low hydro gen overvoltage state having a hydrogen overvoltage lower than that of the cathode of from 0.015 to 0.25 volt selected from the group consisting of platinum, palladium, rhodium, iridium, and nickel, subjecting said cathode including said selected areas to plating current density whereby said chromium is deposited onto said predetermined areas and said selected areas remain stopped-off.

7. The process for electroplating chromium from a chromium plating bath as claimed in claim 6 wherein said bath is an aqueous chromium plating bath containing sulfate ions.

8. The process for electroplating chromium from a chromium plating bath as claimed in claim 6 wherein said bath is an aqueous chromic acid bath containing sulfate ions and complex fluoride ions.

9. The process for electroplating chromium from a chromium plating bath as claimed in claim 6 wherein said metal in low hydrogen overvoltage state is platinum.

10. The process for electroplating chromium from a chromium plating bath as claimed in claim 6 wherein said metal in low hydrogen overvoltage state is palladium.

11. The process for electroplating chromium from a chromium plating bath as claimed in claim 6 wherein said metal in low hydrogen overvoltage state is rhodium.

12. The process for electroplating chromium from a chromium plating bath as claimed in claim 6 wherein said metal in low hydrogen overvoltage state is iridium.

13. The process for electroplating chromium from a chromium plating bath as claimed in claim 6 wherein said metal in low hydrogen overvoltage state is nickel.

14. The process for electroplating a plate from a bath onto substantially the entire predetermined surface area of a cathode piece which comprises placing immediately adjacent to said surface and said piece in initimate electrical contact therewith an auxiliary predetermined piece, placing immediately adjacent to said auxiliary predetermined piece and in intimate electrical contact therewith a selected area piece which is in intimate electrical contact with a metal in low hydrogen overvoltage state having a hydrogen overvoltage lower than that of the cathode of from 0.015 to 0.25 volt selected from the group consisting of platinum, palladium, rhodium, iridium, and nickel, said selected area piece being spaced from said cathode piece, subjecting said cathode including said selected areas to plating current density in a plating bath whereby said plate is deposited onto said predetermined areas and said selected areas remain stopped off.

15. The process for electroplating a plate from a bath having a pH below about 5 onto a cathode which is supported during plating on an electrically conductive rack which comprises placing at least a portion of said electrically conductive rack in intimate electrical contact with a metal in low hydrogen overvoltage state having a hydrogen overvoltage lower than that of the cathode of from 0.015 to 0.25 volt selected from the group consisting of platinum, palladium, rhodium, iridium, and nickel, and subjecting said cathode including said rack to plating current density whereby said plate is deposited on said cathode and said portions of said electrically conductive rack remain free of plate.

References Cited UNITED STATES PATENTS 1,750,418 3/1930 McFarland 20415 2,048,578 7/1936 Van Der Horst 204-23 3,041,257 6/1962 Cope et al. 2 045l OTHER REFERENCES The Corrosion Handbook: H. Uhlig, 1948, p. 1145, John Wiley.

JOHN H. MACK, Primary Examiner T. TUFARIELLO, Assistant Examiner US. Cl. X.R. 204-18 

